Aluminide coatings have been well known for a number of years and are widely used to protect metallic surfaces from oxidation and corrosion. Aluminide coatings are widely used in gas turbine engines because they are economical and add little to the weight of the part. Aluminide coatings are applied by a pack diffusion (or pack cementation) process. Other coatings are also applied by pack processes including silicon and chromium as well as alloys based on aluminum, silicon, and chromium. Hereinafter, except were indicated, the term aluminide will be understood to encompass diffusion coatings based on aluminum, silicon, chromium and alloys and mixtures thereof.
Aluminide coatings are formed by diffusing aluminum into the surface of the superalloy article to produce an aluminum-rich surface layer which is resistant to oxidation. Superalloys are high-temperature materials based on nickel or cobalt. Examplary patents showing diffusion aluminide coating processes include U.S. Pat. No.: 3,625,750, U.S. Pat. No.: 3,837,901, and U.S. Pat. No.: 4,004,047 which are incorporated herein by reference. Typically, aluminide coatings are applied by a pack process. In a pack process a powder mixture including an inert ceramic material, a source of aluminum, and a halide activating compound is employed. The powder materials are well mixed and the parts to be coated are buried in the powder mix. During the coating process an inert or reducing gas is flowed through the pack and the pack is heated to an elevated temperature.
The pack coating process involves complex chemical reactions in which the halide activator reacts with the aluminum source to produce an aluminum-halide compound vapor which contacts the surface of the part. When the vapor contacts the superalloy surface it decomposes, leaving the aluminum on the surface while the halide is released to return to the aluminum source and continue the transport process. After the aluminum is deposited on the superalloy surface, it diffuses into the substrate. Diffusion is promoted by conducting the process at elevated temperatures, typically in the order of 1,500.degree. F. to 2,000.degree. F. In the case of silicon and chromium-based coatings, similar reactions occur.
In the case of nickel-base superalloys, which are the most widely used type of superalloys, and which are used extensively in gas turbine engines, the predominant material found in the aluminide layer is NiAl which is formed near the surface. Other nickel aluminum compounds are often found further below the surface as are compounds between aluminum and the alloy elements in superalloy, including e.g., cobalt, chromium, titanium, and refractory materials such as tungsten, tantalum, and molybdenum. In the case of chromium-based coatings, a chromium enriched surface layer forms while in the case of silicon-based coatings silicide compounds form.
In gas turbine engines the high turbine blades are invariably air-cooled to permit operation of the engine at higher temperatures. The cooling air is derived from air which is pressurized by the compressor section of the engine. As engine operating conditions increase in more modern engines, the temperature of the cooling air has increased to the point where such "cooling" air may actually have temperatures as high as 600.degree. F. to 1,100.degree. F. It has been observed that such high temperature cooling air causes undesirable oxidation on the internal cooling passages of the turbine blades and other air-cooled gas turbine engine hardware. Other gas turbine hardware made of superalloys, which also contain cooling holes and may be coated according to the present invention. These include vanes and air seals.
Thus, it is desired to coat the internal passages and cooling holes in the blade with the aluminide coating so as to reduce oxidation. These holes typically have a diameter from about 0.010 inches to about 0.025 inches and a depth of typically from about 0.030 inches to about 0.300 inches. The cooling holes are of a small diameter to improve cooling efficiency.
A significant practical problem is encountered in the pack coating of gas turbine engine hardware having such fine holes. At the conclusion of the coating process, the particulate material in the coating pack is found to be firmly packed in the fine passageways. Microscopic examination suggests that the fine particulate material is sintered together and to the walls of the passageways during the coating process, and probably during the cooling cycle from the coating process, by a reaction involving the halide activating material. In addition, the difference in the coefficient of thermal expansion between the particulate pack coating material which is mainly a ceramic material and the superalloy article is fairly large. It is possible this differential thermal contraction may contribute to the packing process.
In any event, removal of the material from the cooling holes after coating is a major problem. Various schemes such as chemical dissolution, grit blasting, and mechanical means are employed. Most commonly, hand removal of the powder material is performed. Since each blade may contain 100 to 300 cooling holes, the time required to probe each passageway with a thin piano wire probe to remove the sintered pack material is significant. Further, even assuming that the time was not a factor, it is often found that the material can simply not be removed by mechanical means and that the holes must be redrilled (and of course, the redrilled holes will not have a protective coating on their walls).